Probe for measuring carbon flux

ABSTRACT

A probe (14) comprises a tubular electrical conductor (15) supported by a sleeve of insulating material (16). The two ends of the conductor are connected to couplings (23, 24) for the circulation of a reducing gas through the entire length of the conductor, as well as to an electric circuit (20) kept at a constant voltage and hence permitting measurement of the resistance in a segment (15c) of the conductor which is exposed to the atmosphere of a furnace, the resistivity of the segment varying as a function of the carbon content of the wall of the conductor.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to case-hardening technology, and moreparticularly to a method of monitoring a heat treatment with carbonpenetration, carried out on workpieces placed in a furnace, of the typecomprising the use of a probe for measuring the electrical resistance ofa conductor placed in the presence of the atmosphere of the furnace, andthe processing of the resistance values successively obtained by thesemeasurements. The invention further relates to a probe for carrying outthis method.

2. Description of the Related Art

The various developments which have taken place during the past fewyears in the art of gaseous cementation have pertained essentially toreducing the cycle times, to the quality and reproducibility of thetreatments, as well as to the gas consumption and improvement of thesafety conditions.

This research has led to supersaturating atmospheres, often out ofequilibrium, maintained in agitated reactors, used at atmosphericpressure or under partial pressure of hydrocarbons in a vacuum furnace.

The values of the carbon potential carried out by indirect measurement,based on residual gases such as H₂ O, CO₂, or O₂, can no longer bevalidly defined.

In treatment under vacuum or in a supersaturating atmosphere, where therelease of the carbon does not take place via cracking of the COmolecule but by the direct decomposition of a hydrocarbon, the conceptof equilibrium potential no longer exists and could be replaced by theterm "kinetic potential," i.e., enrichment according to a linear law (inthe usual range of cementation) as a function of time.

A method of monitoring and regulating completely adapted for followingthese reactions is the direct measurement of the carbon flux.

The principle consists in following in real time the carbon which entersat the surface of the measuring element, either by weighing(thermobalance or strip system) or by gas analysis after secondaryreaction of the carbon with a gas in an system isolated from thereactor, or by measurement of the resistance of a detector as a functionof its carbon enrichment.

The first principle, based on weighing an element, can hardly be used inan industrial reactor, for the electronic balance is very sensitive tojolts and vibrations. As for measurement on foil, the values are onlyintermittent and indicative of a state during a brief period of time.

The second method, devised by Meyer and Schmidt, consists in using acarbon-flux probe in which the carbon diffuses from the atmosphere ofthe furnace into a thin-walled steel tube; a decarburizing atmospherebased on humid N₂ and H₂ circulates within the tube. The carbon flux isdetermined from the CO/CO₂ content of this atmosphere (see U.S. Pat. No.3,843,419, for example).

The main drawback of this system is that the analysis does not takeplace in situ and the measurement chain based on infrared analyzersbecomes complex and inaccurate.

The other system, developed by Joachin Wunning, uses the resistancemethod: a detector in the form of a very fine, short wire is enriched inthe atmosphere of the furnace, and the measurement of its electricalresistance gives an indication of its carbon content, hence of thecarbon flux as a function of time. At regular intervals, the probe isdecarburized by an injection of H₂, H₂ O, and N₂ around the detector (J.Wunning, Die C-Stromregelung bei der Gasaufkohlung, HTM 40, 1985).

This technique cannot be used in a vacuum reactor, for the introductionof a decarburizing gas shifts the gaseous reactions, weakens theenrichment kinetics, causes intergranular oxidation at the surface ofthe workpieces, and increases the working pressure.

The fragility of the wire and the complexity of its assembly afterbreakage (for soldering) must likewise be noted.

U.S. Pat. No. 2,935,866 to Schmidt and Wunning also describes a methodof measurement using the electrical resistance of conductors. In thiscase, one detector is placed in an atmosphere having a known carboncontent and another in the atmosphere to be measured.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved method andprobe which eliminate these drawbacks.

To this end, in the method according to the present invention, of thetype initially mentioned, a tubular-shaped conductor is used, one of thefaces of a segment of the conductor is placed in contact with theatmosphere of the furnace, and the space adjoining the other face of thesegment is connected to a supply of reducing gas.

The probe according to the present invention comprises a U-shapedtubular detector made of a ferrous metal, the two ends of which passthrough a core of insulating material and are connected electrically toa voltage source by a connection which passes through an instrument formeasuring electrical resistance, on the one hand, and, from the point ofview of the flow of the fluids, to a feed supply and to an exhaust ofreducing gas, on the other hand.

The principle of this invention consists in using a tubular detector inwhich there circulates a decarburizing atmosphere on the basis of N₂,H₂, H₂ O, and CO₂. This tube being placed partially in the atmosphere ofthe furnace, and its wall being very thin, the carbon diffuses throughand combines with the H₂ O to be exhausted from the system.

The carbon flux is measured by the variation in resistivity of thedetector. Knowing the rate of gas flow and the length and surface of thedetector, the measurement of this resistance permits a continuousanalysis, and the advantages are numerous:

the possibility of working in vacuum reactors and in anoxidizing-reducing gas atmosphere;

a continuous and precise method of analysis, for it is carried out inthe reactor;

easy interchangeability (by electrodes);

reliability and flexibility of use;

analysis based on an electric signal without processing making use ofinfrared means;

convenient control of an installation based on this system;

rapid response time.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the inventive method and probe will now bedescribed in detail with reference to the accompanying drawings, inwhich:

FIG. 1 is a diagram of a prior art probe,

FIG. 2 is a graph illustrating the operation of the probe of FIG. 1,

FIG. 3 is a diagram, partly in section, of a furnace equipped with aprobe according to the present invention for carrying out the inventivemethod,

FIG. 4 is a section on a larger scale of the probe shown in FIG. 3,

FIG. 5 is a sectional diagram on a still larger scale for explaining thecourse of the method, showing a segment of the tubular conductor visiblein FIG. 4,

FIG. 6 is a diagrammatic view showing in section a segment of the wallof the tubular conductor, and

FIG. 7 is a graph showing the evolution of the carbon content in thesurface zone of the workpieces placed in the furnace of FIG. 3.

FIGS. 1 and 2 illustrate the aforementioned prior art method developedby Wunning for monitoring a cementation heat treatment. A probe 1 withinan envelope 2 passing through the wall 3 of a furnace comprises aU-shaped ferrous metal wire 4, the bend of which constitutes a resistor5. Probe 1 further comprises a temperature-measurement thermocouple 6. Acalculating unit 7 is designed to control the sending of a currentacross resistor 5, to measure periodically the value of the resistance,to control as well, by means of a valve 8, a feed of decarburizing gasin the space within envelope 2, and finally to control by means of avalve 9 the flow of cementation gas entering the furnace.

For carrying out this process, a calculation program based upon theresistance values measured on segment 5 of wire 4 was developed. As thissegment of wire, placed in the furnace, has a tendency to become rapidlysaturated with carbon, whereas the carbon content of the surface zone ofthe workpieces to be case-hardened increases gradually as the carbondiffuses to the interior of the workpieces, the prior art methodconsists in decarburizing wire 4 periodically, so that the carboncontent of this wire, as a function of time, yields a zigzag curve 10(FIG. 2), whereas the carbon content of the surface zone of theworkpieces yields a continuous curve 11. The intervals during whichdecarburization is carried out are designated as "1" in FIG. 2.

As may be seen, the decarburization of wire 4 necessitates theintroduction of the decarburizing gas within the furnace, so that thistechnique cannot be used in a vacuum reactor and presents drawbacks evenwhen the atmosphere of the furnace has pressure.

FIG. 3 therefore illustrates in a general way the arrangement by meansof which the method to be described below may be carried out. A furnace12 contains a charge 13 of workpieces to be case-hardened. A probe 14comprises a detector in the form of a U-shaped tubular electricalconductor 15 disposed within a support sleeve 16. The two ends oftubular conductor 15 are connected to feed and exhaust pipes 17 so thata reducing gas can circulate within the conductor. This circulation iscontrolled by one or more valves 18, which are in turn controlled from acontrol unit 19. The circulation of an electric current in tubularconductor 15 via a circuit 20, and the measurement of the electricalresistance of conductor 15, are also effected by control unit 19.

A feed device 21, controlled by a valve 22, allows the cementation gasto be introduced into furnace 12.

FIG. 4 shows the structure of probe 14 in more detail. Tubular conductor15 comprises two arms 15a and 15b which extend parallel to the inside ofsleeve 16, the latter being a part serving as a support, as electricaland thermal insulation, and as protection for conductor 15. A shortsegment 15c of conductor 15, of a length L, is exposed to the atmosphereof the furnace and constitutes the detector proper. In the embodimentbeing described, segment 15c is coiled in the shape of a solenoid. Itmight also be disposed along a serpentine line or any other layoutoffering a large surface in contact with the atmosphere of the furnacetogether with compactness and unobstructed circulation. The metal ofwhich conductor 15 is made is a ferrous metal of a composition such thatmeasurement of its electrical resistance makes it possible to determinethe average carbon content in the thickness of the conductor wall. Thiswall will be thin enough, e.g., about 0.1 mm, so that the carbon maydiffuse rapidly through it and a state of dynamic equilibrium be easilybrought about, as will be seen below.

Arms 15a and 15b of tubular conductor 15 are preferably copper-coated onthe outside in order to reduce to a minimum the drop in voltage and themodification of resistance between the ends of the two arms 15a and 15band segment 15c. These two ends are, in fact, connected outside thefurnace both to couplings 23 and 24 leading to reducing gas circuit 17,and electrically to circuit 20.

The method described may be carried out during the cementation operationeither continuously or intermittently. FIGS. 5 and 6 illustrate thephenomenon to be monitored and give the physical magnitudes whose valuesare to be processed by the computer. FIG. 5 shows a short tubularsegment representing a portion of conductor 15 with its thin cylindricalwall. If the outside surface of this tubular segment is exposed to acarburizing atmosphere at a given temperature T, the carbon penetratesinto the metal of the conductor, and the quantity of carbon brought inis represented by the term mV₁. This carbon diffuses through the wall;and as the interior space of the tubular conductor is in the presence ofa reducing gas which circulates along the length of the conductor, thisgas exhausts the carbon, the quantity of carbon exhausted beingrepresented by the term mV₂.

FIG. 6 illustrates the situation of equilibrium which is thusestablished. On the graph, the carbon content is given on the y-axis andthe distance through the wall of conductor 15c on the x-axis. The carboncontent on the outside surface of this wall is greater than on itsinside surface, and the difference Δ%C corresponds to a transfer of massof carbon m (in grams per sq.m. per hour).

The value of the resistance of conductor segment 15c depends upontemperature T, length L, thickness ΔX, and the coefficient ofresistivity, which in turn depends upon the carbon content. Thus thevalue of the resistance at any given moment t makes it possible todetermine at that moment the average carbon content %C in conductor wall15c and the evolution of that value in the course of time, taking intoaccount the rates of flow of carburizing gas and of reducing gas mV₁ andmV₂. It permits determining the degree of evolution of the phenomenon ofpenetration of carbon into the workpieces and, consequently, to followtheir carburization as shown in FIG. 7.

When the conditions of equilibrium are achieved, the interior space ofthe tube comprises a constant carbon potential. A constant flux ofcarbon is absorbed by the constant flow of the reducing gas.

What is claimed is:
 1. A probe for measuring carbon flux, comprising:aU-shaped tubular detector of a ferrous metal, said detector comprised ofU-arms each having an end and a U-bend, a core of insulating materialsupporting said detector, a voltage source, an instrument for measuringelectrical resistance, means for supplying a reducing gas, and means forexhausting said reducing gas, the ends of said U-arms of said detectorbeing electrically connected to said voltage source across saidinstrument and fluidly connected to said means for supplying and forexhausting said reducing gas.
 2. The probe of claim 1, furthercomprising a sleeve of protective insulating material, the U-arms ofsaid detector being encased in said sleeve, and the U-bend of saiddetector being a serpentine-shaped conductor segment exposed to theatmosphere of a furnace.
 3. The probe of claim 1, further comprising asleeve of protective insulating material, the U-arms of said detectorbeing encased in said sleeve, and the U-bend of said detector being asolenoid-shaped conductor segment exposed to the atmosphere of afurnace.
 4. The probe of claim 1, further comprising a sleeve ofprotective insulating material, said U-arms of said detector beingcoated with a layer of copper within said sleeve.